The purpose of this study is to evaluate the safety and pharmacokinetics of single and multiple doses of PMPA pro-drug administered orally to HIV- infected patients and to evaluate the anti-HIV activity of PMPA pro-drug, as demonstrated by increases in CD4 cell counts and decreases in HIV RNA, when administered orally as a single dose and daily for four weeks to HIV- infected patients with CD4 cell counts >200 cells/mm3.